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Cis/Trans in Newman projections

TexasOMFS

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  1. Dental Student
    Hmm typically Newman projections are used for looking down a C-C bond, usually of a n-alkane. Cis/Trans designation is for alkenes or possibly cycloalkanes. You wouldn't ever have to designate Cis or Trans for a cycloalkane based on the Newman projections. Typically, the projections are just used to map relative stabilities of different conformations of the molecule based on steric and eclipsing interactions.
     
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    rippinitez

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    1. Pre-Dental
      I obviously can't post the actual problem from the book. But here is an example I made up that is also "trans".

      Also, can you tell from this example whether the H and Br are anti?
       

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      darkwalk

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        This one is tricky. I don't think you'll get something like this in the DAT, but doesn't hurt to know.

        Using the priority rules:
        http://en.wikipedia.org/wiki/Cahn_Ingold_Prelog_priority_rules

        The carbon closest to the viewer is R.
        1 is bonded to Br, so that takes priority
        2 is bonded to 2 real carbons and 1 ghost carbon, taking priority away from 3 which is bonded to 2 carbons and 1 H.

        The carbon away from the viewer is R.
        The first atom attached are C,C,C,H, so we must look at the atoms attached to them
        1 is attached to Br, so that must take priority regardless of the double bond in 2
        2 takes priority compared to 3 because it has double bond, therefore, replace with "ghost" atoms
         

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        darkwalk

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          The proton being attacked must be anti with Br.

          Since you have two phenyl group, I assume they must be anti with each other due to strain. (I'm not 100% positive about this)

          http://en.wikipedia.org/wiki/Gauche_effect

          If they are anti, Br and H are not, so E2 cannot take place.

          However, if enough energy exists to Gauche the structure, then E2 can take place.

          Anyone want to chime in?

          I think this question is great for studying, but I don't think you will encounter such difficult question.
           
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